Our major objectives are to examine cell replication and repair of DNA damage as a function of aging in human cells in vitro and in rat and mouse tissues in vivo. We have continued our studies of skin fibroblast cultures derived from human volunteer subjects and found additional evidence for a loss of replicative ability with in vivo aging. A technique has been developed to examine cell replication in vivo in intact animals utilizing the BrdU-differential staining technique. Preliminary results obtained with this technique indicate a decline in cell replication as a function of aging. Further research will be directed not only at defining the decline in cell replication but also at investigating the mechanisms for this functional loss. The BrdU-differential staining techniques have also been applied to examining DNA repair by analyzing the frequencies of sister chromatid exchanges (SCE) induced by various DNA damaging agents (i.e., mitomycin C) (MMC) in vivo in mouse and rat bone marrow cells and in vitro in cultured human fibroblasts. In all these systems, a significant decrease in MMC-induced SCE was observed in the older cell populations. The mechanism for the altered response to DNA damage as manifested by diminished SCE will be investigated. BIBLIOGRAPHIC REFERENCES: Schneider, E.L. and Fowlkes, B.J.: Measurement of DNA content and cell volume in senescent human fibroblasts utilizing flow multiparameter single cell analysis. Exp. Cell Res. 98: 298-303, 1976. Schneider, E.L., Sternberg, H. and Tice, R.: In vivo analysis of cellular replication. Proc. Natl. Acad. Sci. (USA) 74: 2041-2044, 1977.